(a) Field of the Invention
The present invention relates to a diagnostic system of a fuel cell stack that diagnoses a state and/or a failure of the fuel cell stack.
(b) Description of the Related Art
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used. Fuel cells are different from batteries in that they require a constant source of fuel and oxygen to run, but they can produce electricity continually for as long as these inputs are supplied Fuel cells may be applied to supply industrial, household, and vehicle driving power as well as to supply power to a small-sized electric/electronic product.
For example, one of the ways that a vehicle can be powered is by a fuel cell system such as a polymer electrolyte membrane fuel cell or a proton exchange membrane fuel cell (PEMFC). These types of fuel cells have a higher power density, fast starting time and fast power conversion reaction time at lower operational temperatures.
PEMFCs typically include a membrane electrode assembly (MEA) in which a catalyst electrode layer in which an electrochemical reaction occurs is attached to both sides of a solid polymer electrolyte film in which hydrogen ions move. Also included in the PEMFC, is a gas diffusion layer (GDL) that uniformly distributes reaction gases and transfers generated electrical energy through the cell. A gasket and an engaging device are also typically provided. The engaging device maintains an appropriate engaging pressure and airtightness of reaction gases and coolant. Also a bipolar plate is also provided to move reaction gases and coolant through the cell.
When assembling a fuel cell stack the gas diffusion layer and the MEA are disposed in the middle of the cell. As a result, the catalyst electrode layers of the MEA, i.e., an anode and a cathode to which a catalyst is applied so that hydrogen and oxygen may react at both surfaces of a polymer electrolyte film, are on the outer surfaces of the MEA. Then the gas diffusion layer and a gasket are stacked on top of the anode and the cathode respectively.
On the outer surface of the gas diffusion layer, a reaction gas (typically hydrogen as a fuel and oxygen or air as an oxidizing agent) is supplied, and a bipolar plate having a flow field through which coolant passes is placed thereon. By forming such a configuration in a unit cell, after a plurality of unit cells are stacked, an end plate for supporting a current collector, an insulation plate, and stacking cells are coupled on the outermost surfaces of the stack. By repeatedly stacking and engaging unit cells between the end plates, a fuel cell stack may be formed.
In order to obtain a potential necessary for a vehicle to be operated, unit cells should be stacked accordingly a necessary potential, in order to ensure that a sufficient potential is output by the cells.
The potential generated by each unit cell is typically about 1.3 V. Thus, in order to generate power that is necessary to power a vehicle, a significant number of cells must be stacked in series. Thus, determining during a failure which cell is not working appropriately can be time consuming and at times difficult. Thus, fuel cell vehicles require a diagnostic system to identify and determine individual unit failures.
FIG. 1 is a schematic diagram of a diagnostic system of a fuel cell stack according to an exemplary embodiment of the conventional art. Referring to FIG. 1, the diagnostic system of the fuel cell stack according to the exemplary embodiment of the conventional art includes an alternating current (AC) current injector 20 that injects a diagnostic AC current into a fuel cell stack 10, and a diagnostic analyzer 30 that performs diagnostics on the fuel cell stack 10 by analyzing the change in an AC current as result of the injection of the diagnostic AC current.
Because these types of diagnostic systems generally perform diagnosis through total harmonic distortion analysis (THDA) of a diagnostic AC current signal, the diagnostic analyzer 30 typically includes a harmonic analyzer.
When a diagnostic AC current IAC is injected into the fuel cell stack 10 by the AC current injector 20, the diagnostic AC current IAC is overlapped with a current ISTACK of the fuel cell stack 10. Therefore, a diagnostic AC current IAC component is also included in a current ILOAD flowing to a load 40.
When the current ISTACK of the fuel cell stack 10 and the diagnostic AC current IAC of the AC current injector 20 are overlapped and thus reach the load 40, the diagnostic analyzer 30 detects a voltage from the fuel cell stack 10, converts and analyzes a frequency of the detected voltage, and diagnoses a state and/or a failure of the fuel cell stack 10.
However, in order to prevent collision with a DC current from the fuel cell stack 10, the AC current injector 20 of a diagnostic system of an exemplary embodiment of the conventional art also generally includes a decoupling capacitor (CT). Because the CT of the AC current injector 20 should pass through a lower frequency of AC current, the CT should have a considerably large capacity. Therefore, the CT of the AC current injector 20 is formed by coupling multiple small capacity capacitors (CN) in parallel. However, due to the large quantity of these capacitors that is required, the overall size of the CT and the cost is greater than is desirable by most automotive manufactures. Additionally, when a diagnostic AC current of the AC current injector 20 passes through the CT, the diagnostic AC signal may be distorted and thus precise diagnostic may not be performed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.